it's hard to say without knowing your site, niche and keywords. It could be any, or more / all of these:
- your content sucks
- your site is bad / not well optimized
- you might have noindex tags on important pages, which will prevent you from getting indexed and ranking
- you might be lacking proper siloing and internal linking (which is very important nowadays)
- your topical authority could be all over the place (this issue is particularly prevalent with newbies that create sites around all sorts of topics, thus diluting their topical authority so much that google doesn't consider those sites relevant to anything, therefore they don't rank them)
A lot of reasons, really... and we can't provide a solution until we know your niche, site and keywords, like I said (but don't make your site public, it's risky)
However, if I understand you correctly, you are ranking for long tail keywords, but not for main keyword, which sounds like your page lacks backlinks, both internal ones and external ones.
On top of this, the page might not be well optimized for your main keyword, otherwise I can't figure out why your long tails rank but your main keyword doesn't.
If you are indeed ranking for those long tails, but not for your main keyword, check your content with a content optimization tool (Frase, RankMath, etc) and follow the suggestions of these tools to optimize the content with the right LSI terms and SEO entities, and then point some internal links to this page using exact / partial match anchors, and also build some external QUALITY links (2-3 for starters to see what happens) to this page using 1-2 exact / partial match anchors...
